A recording density of a hard disk (HDD), a main recording assembly of a personal computer, is being increased at a remarkable rate, 100-fold or more in 10 years. In recent years, HDDs are being developed not only for personal computers, but also for digital household electric appliances and portable information stations, so that a further increase in recording density is wanted for these uses.
At the moment, conventional HDDs employ longitudinal recording systems in which magnetization is kept in the direction of the disk plane. In the longitudinal recording system, the magnetic recording layer should be made thinner to suppress diamagnetism in the magnetic domains and to form a strong magnetic field above the medium. The individual magnetic particles in the magnetic layer constituting the thinner magnetic recording layer are smaller in size, and magnetic energy held by the smaller magnetic particles is affected significantly by thermal energy. Consequently, the effect of superparamagnetism (thermal fluctuation) becomes remarkable, which causes a loss of the recorded magnetization.
On the other hand, in the vertical recording system in which the magnetization is kept in a direction perpendicular to the disk plane, with the increase of the recording density, demagnetization is decreased and the magnetization is stabilized in principle, being different from the longitudinal recording system, and the superparamagnetism would be less liable to occur. Therefore, the vertical magnetic recording system is attracting attention as a technique for increasing the density of the magnetic recording.
At present, CoCr alloys are mainly studied as the material of the recording layer of the vertical magnetic recording medium. For more effective suppression of the superparamagnetism, FePt alloys are attracting attention, which alloys have an L10 ordered alloy structure and which are capable of generating a strong anisotropic magnetic field of 7×107 erg.cm−3 and of achieving a greater coercive force, as a new-generation recording film material.
For production of the FePt alloy, many investigations are conducted regarding a vapor-quenching process, such as sputtering, and vapor deposition. In the FePt alloy production by the vapor-quenching process, an fcc phase as an irregular phase is formed first, and is then transformed into an L10 structure with crystal grain growth by a high temperature treatment at 600° C. or higher. However, for lower noise in the magnetic recording medium, the crystal grains should be made finer. Further, the high temperature treatment in the magnetic recording medium production process is not suitable because of undesired melting of the substrate and other disadvantages.
For formation of an ordered phase of an L10 structure of FePt by a heat treatment below 600° C., the following methods are disclosed: a method of alternate lamination of monoatomic layers of Fe and Pt (001) by molecular beam epitaxy (MBE) or the like process to prepare an FePt magnetic material having a strong coercive force, and a method of production of an FePt magnetic material having a coercive force of about 5000 Oe by adding a third element, such as Cu and Ag.
As another process, a liquid phase process of electrolytic plating is disclosed for the FePt magnetic material production. The electrolytic plating process, which does not use an expensive vacuum apparatus, such as a sputtering apparatus and a vapor deposition apparatus, is suitable for industrial mass production (Japanese Patent Application Laid-Open No. 2002-180259).
A production process for an FePt magnetic material by electrolytic plating is disclosed in which an FePt electric deposition film is produced from iron sulfate and hexachloroplatinate(IV) salt (intermag 2003 “electrodeposited FePt film”).
However, in the conventional plating solution, the ionic Fe and the ionic Pt are unstable, tending to form more stable precipitates. Thereby, the intended FePt is not readily obtainable. A stable plating solution is wanted for the FePt plating.
With the above background, the present invention provides a plating solution for FePt plating.
The present invention also provides a process for producing a structure by use of the plating solution for FePt plating.
The present invention further provides an apparatus, which employs the plating solution for FePt plating.
The aforementioned problems are solved by the means described below.
According to an aspect of the present invention, there is provided a plating solution, which contains ionic Fe, ionic Pt, and a complex agent, at a molar ratio of the ionic Fe to the ionic Pt ranging from 0.75 to 3.
The complex agent preferably contains tartrate ions or citrate ions.
The concentration of the ionic Fe preferably ranges from 0.005 mol/L to 0.1 mol/L.
The plating solution preferably has a pH ranging from 5.0 to 10.5.
The ionic Fe and the ionic Pt preferably form a double complex constituted of an Fe complex and a Pt complex.
The plating solution preferably contains ionic Cu and a complex agent for the ionic Cu.
According to another aspect of the present invention, there is provided a process for producing a structure comprising steps of:
providing an electrode and an object to be plated in a vessel containing the plating solution, and plating the object with a magnetic material containing FePt from the plating solution by applying voltage to the electrode to form a structure.
The structure formed in the process is preferably heat-treated further at a temperature ranging from 450° C. to 750° C.
The structure in the process is preferably heat-treated further in the presence of hydrogen.
The object to be plated in the process is preferably a structure having holes, and the step of plating the object to form the structure is deposition of the magnetic material containing FePt into the holes. According to still another aspect of the present invention, there is provided an apparatus, having the plating solution, a vessel for holding the plating solution, and electrodes, for conducting plating by application of a voltage to the electrodes.According to the present invention, the plating solution for FePt plating is stabilized by adjusting the molar concentration ratio of the ionic Fe to the ionic Pt within the range from 0.75 to 3 in the plating solution (the ratio is referred to occasionally as a “FePt ratio”). The use of this plating solution enables formation of an FePt plating layer having a strong coercive force. A structure having holes filled with the FePt is useful for forming a magnetic recording medium for high-density recording.